In a power transmission or distribution network, switching apparatuses are incorporated into the network to provide automatic protection in response to abnormal load conditions or to permit opening or closing (switching) of sections of the network. The switching apparatus may therefore be called upon to perform a number of different operations such as interruption of terminal faults or short line faults, interruption of small inductive currents, interruption of capacitive currents, out-of-phase switching or no-load switching, all of which operations are well known to a person skilled in the art.
In switching apparatuses the actual opening or closing operation is carried out by two contacts where normally one is stationary and the other is mobile. The mobile contact is operated by an operating device which comprises an actuator and a mechanism, where said mechanism operatively connects the actuator to the mobile contact.
Actuators of known operating devices for medium and high voltage switches and circuit breakers are of the spring operated, the hydraulic or the electromagnetic type. In the following, operating devices will be described operating a circuit breaker but similar known operating devices may also operate switches.
A spring operated actuator, or spring drive unit as it is also called, generally uses two springs for operating the circuit breaker; an opening spring for opening the circuit breaker and a closing spring for closing the circuit breaker and reloading the opening spring. Instead of just one spring for each one of the opening spring and the closing spring, sometimes a set of springs may be used for each one of the opening spring and the closing spring. For example, such a set of springs may include a small spring arranged inside a larger spring or two springs arranged in parallel, side by side. In the following, it should be understood that when reference is made to the spring of the respective opening spring and the closing spring, such a spring could include a set of springs. Another mechanism converts the motion of the springs into a translation movement of the mobile contact. In its closed position in a network the mobile contact and the stationary contact of the circuit breaker are in contact with each other and the opening spring and the closing spring of the operating device are charged. Upon an opening command the opening spring opens the circuit breaker, separating the contacts. Upon a closing command the closing spring closes the circuit breaker and, at the same time, charges the opening spring. The opening spring is now ready to perform a second opening operation if necessary. When the closing spring has closed the circuit breaker, the electrical motor in the operating device recharges the closing spring. This recharging operation takes several seconds.
Illustrative examples of spring operated actuators for a circuit breaker can be found e.g. in U.S. Pat. No. 4,678,877, U.S. Pat. No. 5,280,258, U.S. Pat. No. 5,571,255, U.S. Pat. No. 6,444,934 and U.S. Pat. No. 6,667,452.
In known spring operated actuators axially acting springs, i.e. compression or tension helical springs are used. Also torsion springs such as torsion bars, helical springs and clock springs are used for the actuation of the opening and closing movements.
The use of axially acting springs is an arrangement that requires much space, in particular since the springs normally are directed at an angle to the drive shaft. Furthermore, these types of springs require mechanisms for converting the linear spring movements to rotational movements of the drive shaft. This increases the required number of moving parts in the actuator and thus makes it complicated.
Torsion springs are less frequently used for the actuators. Traditionally also these springs are located at an angle to the drive shaft or axially offset from the drive shaft. Known torsion spring operated actuators also have the axes of the opening spring and the closing spring axially offset in relation to each other.
Known spring operated actuators, whether employing axially acting or torsion springs, thus suffer from the drawbacks that they require much space and a relatively large amount of components.
With the term “end” related to a helical torsion spring is in this application meant the end of the spring material, i.e. the end in the direction of the spring helix. For the ends in the axial direction the term “axial end” is used.